GtkCurve/GtkGammaCurve and ENABLE_BROKEN

[Owen Taylor <otaylor redhat com>]

I'd like to protect the headers for GtkCurve and GtkGammaCurve under
#ifndef G_ENABLE_BROKEN.

Reasons:

 - They aren't used much
 - When people use them they don't do what they want (they aren't
   plotting widgets)
 - They are ugly
 - They are not at all robust and a lot of things break if you
   use them not in exactly the way they are intended 
   (#57012, #57105)
 - I don't want to fix them.

The few places that use them (xsane, gsumi, anything else?) would
still have them there for a while yet, but this would be an 
effective discouragement for anything else to use them.

This might just be laziness though...

                                           Owen